1. Technical Field
This invention relates generally to spark ignition devices, such as spark plugs for internal combustion engines, and more particularly to spark ignition devices having heater elements.
2. Related Art
In construction of an ignition device for an internal combustion engine, such as for a spark plug, a compromise generally needs to be made between selecting the operating heat range at which the spark plug will operate. On one hand, if the temperature selected is too hot, the spark plug will typically have a reduced life and can ultimately reduce the life of engine components. On the other hand, if the temperature selected is too cold, the spark plug may exhibit a tendency to become fouled via carbon deposits on an insulator of the spark plug, thereby resulting in reduced performance and ultimate failure of the spark plug. Accordingly, it is customary to try to design the spark plug to operate at the hottest temperature possible without greatly impacting the useful life of the spark plug or the engine components. However, this option is not without potential negative consequences in that these spark plugs typically do not operate optimally at cooler operating conditions.
Typically, conventional spark plugs, such as shown in the prior art FIG. 1, have a terminal 1 configured for attachment to a high voltage source. The high voltage travels through the terminal and then through one or more intermediate components to a central electrode 2. The high voltage is insulated from an outer metal shell 3 by an insulator 4. Upon sufficient high voltage reaching the center electrode, a spark jumps from the center electrode to a ground electrode 5 across a spark gap 6, causing ignition of flammable combustion gases. The high voltage current then flows to ground provided by the engine block (not shown) through a threaded region 7 and seat 8 of the metal shell 3 which are in contact with the engine block.
During continued use of the conventional spark plug described above, it is possible for contamination to build up on an exposed outer surface 9 of the insulator core nose which can provide an alternate path for electrical current flow from the central electrode 2. As such, rather than the electrical flow resulting in a spark across the gap 6, the electrical flow jumps directly from the central electrode 2 to the shell 3. This ultimately results in incomplete combustion and failure of the spark plug. Some efforts have been made to overcome the build up of contamination on the outer surface 9 of the core nose, thereby reducing fouling, by increasing the core nose length. The increased length of the core nose increases the operating temperature of the core nose by exposing it to the high operating temperature within the combustion chamber. The increased length core nose is also more resistant to fouling by increasing the distance over which the high voltage must travel. However, increasing the length of the core nose is not without tradeoffs. By extending the tip of the core nose closer to the high temperature within the combustion chamber, the heated core nose tip could inadvertently cause premature ignition of combustion gases within the combustion chamber. In addition, accelerated wear can result to the central electrode, as it must be extended beyond the tip of the extended core nose. Accordingly, continued efforts are made to provide spark plugs with an optimal performance over operating anticipated temperature ranges, while at the same time optimizing the useful life of the spark plugs and associated engine components.